Embodiments of the invention relate generally to structures and methods for packaging semiconductor devices and, more particularly, to a surface-mount packaging structure that includes a diffusion barrier coating.
Surface-mount technology is a method for constructing electronic circuits in which surface mount components or packages are mounted directly onto the surface of printed circuit boards (PCBs) or other similar external circuits. In the industry, surface-mount technology has replaced the through-hole technology construction method of fitting components with wire leads into holes in the circuit board.
One common type of component that is surface-mounted is a power semiconductor device, which is a semiconductor device used as a switch or rectifier in power electronic circuits, such as switched mode power supplies, for example. Most power semiconductor devices are only used in commutation mode (i.e., they are either on or off), and are therefore optimized for this. Many power semiconductor devices are used in high voltage power applications and are designed to carry a large amount of current and support a large voltage. In use, high voltage power semiconductor devices are surface mounted to an external circuit by way of a power overlay (POL) packaging and interconnect system, with the POL package also providing a way to remove the heat generated by the device and protect the device from the external environment.
A standard POL package manufacturing process typically begins with placement of one or more power semiconductor devices onto a dielectric layer by way of an adhesive. Metal interconnects (e.g., copper interconnects) are then electroplated onto the dielectric layer to form a direct metallic connection to the power semiconductor device(s), so as to form a POL sub-module. The metal interconnects may be in the form of a low profile (e.g., less than 200 micrometers thick), planar interconnect structure that provides for formation of an input/output (I/O) system to and from the power semiconductor device(s). The POL sub-module is then soldered to a ceramic substrate (Alumina with DBC, AlN with AMB Cu, etc.) using soldered interconnection for electrical and thermal connectivity. The gaps around the semiconductor between the POL dielectric layer and the ceramic substrate are then filled using a dielectric organic material using either capillary flow (capillary underfill), no-flow underfill or injection molding (molding compounds) to form the POL package.
It is recognized that POL packages are susceptible to moisture, as moisture in the environment may be absorbed by the materials in the POL package. For example the module may absorb moisture within the bulk of Kapton-adhesive layers and organic dielectric material (i.e., underfill, molding compound, etc.) and at the interfaces created by these materials within the package. When soldering the POL module with absorbed moisture to a circuit board, temperatures in the range of 210-260 degrees Celsius are reached and, at these temperatures, the vapor pressure of the moisture in the POL package increases rapidly. This increase in vapor pressure can cause delamination, “pop-corning” and failure if the moisture is excessive. Additionally, in long term storage and use while exposed to moisture, excessive moisture absorption by the POL package and corrosion at dissimilar material interfaces within the package may lead to electrical and mechanical failures due to increased leakage currents and can also cause mechanical damage on interconnects due to swelling of the POL module upon reflow during board-assembly operations.
Long term operating/storage at elevated temperatures in air (or oxygen-rich environments) or with exposure to toxic/corrosive gasses may also affect the long term life and functionality of the POL modules. With oxygen ingress at elevated temperatures, various interfaces can degrade and the mechanical/electrical/thermal performance of the module may be severely affected. As an example, the adhesion between POL metal (Cu) and Kapton is strongly affected by exposure to oxygen at elevated temperatures, with degradation of adhesion strength being seen within 1000 hrs of storage at temperatures between 200 C-250 C. The inclusion of a robust diffusion barrier can slow down the ingress of oxygen (or other degrading gases) and increase the long-term life of the modules.
Therefore, it would be desirable to provide a surface-mount package having a diffusion barrier that reduces the ingress of moisture and gases into the package, so as to provide enhanced reliability to moisture-related and gas-related failure mechanisms. It would further be desirable for such a diffusion barrier to be introduced during various stages of manufacture of the surface-mount package.